In the past aluminum alloys, due to their light weight, have been used for engine blocks for internal combustion engines. To provide the necessary wear resistance for the cylinder bores, it has been customary to chromium plate the cylinder bores, or alternately, to use cast iron liners in the bores. It is difficult to uniformly plate the bores, and as a result, plating is an expensive operation. The use of cast iron liners increases the overall cost of the engine block as well as the weight of the engine.
Hypereutectic aluminum silicon alloys containing from about 16% to 19% by weight of silicon possess good wear resistant properties achieved by the precipitated primary silicon crystals. The conventional aluminum silicon alloy usually contains a substantial amount of copper, generally in the range of 4.0% to 5.0%. Because of the high proportion of copper, the alloy has a relatively wide solidification temperature range in the neighborhood of about 250.degree. F. to 300.degree. F. which severely detracts from the castability of the alloy. The copper also reduces the corrosion resistance of the alloy in salt water environments and thus prevents its use for marine engines.
U.S. Pat. No. 4,603,665 describes an improved hypereutectic aluminum silicon casting alloy having particular use in casting engine blocks, or other components, for marine engines. The alloy of that patent contains by weight from 16% to 19% silicon, up to 1.4% iron, 0.4% to 0.7% magnesium, up to 0.3% manganese, less than 0.37% copper and the balance aluminum. As the copper content is minimized, the aluminum-silicon-copper eutectic is correspondingly eliminated, with the result that the alloy has a relatively narrow solidification range less than 150.degree. F.
Normally the solid phase in a "liquid plus solid" field has either a lower or higher density, but almost never the same density, as the liquid. If the solid phase is less dense than the liquid phase, floatation of the solid phase will result. On the other hand, if the solid phase is more dense, settling of the solid phase will occur. In either case, an increased or widened solidification range will increase the time period for solidification and accentuate the phase separation. With an aluminum silicon alloy the floatation condition prevails and the alloy solidifies with a large mushy zone because of its high thermal conductivity and the absence of the skin formation typical of steel castings. This leads to liquid feeding problems at the micron level during solidification and can also result in significant amounts of microporosity.
When casting large components, such as engine blocks, floatation of primary silicon into the risers of sand castings results in a non-uniform distribution of primary silicon and therefore detracts from the wear resistance of the alloy. For yet unknown reasons, there is a non-uniform distribution of primary silicon in die cast engine blocks.
It is recognized that increasing the silicon content beyond the 16% to 19% range correspondingly widens the solidification range, and as a widening of the solidification range would normally be expected to increase the floatation and contribute to non-uniformity of primary silicon, alloys of higher silicon content have not been candidates for casting engine blocks or engine components.